Method for sharpening rock drill bits



1965 E. G. B. LUNDIN 3,201,903

METHOD FOR SHARPENING ROCK DRILL BITS Filed Nov. 4, 1965 United States Patent O 3,201,903 METHOD FOR SHARPENING ROCK DRILL BITS Erik Gustav Bengt Lundin, Sandvilren, Sweden, assrgnor to Sandvikens Jernverks Aktiebolag, Sandviken, Sweden, a corporation of Sweden Filed Nov. 4, 1963, Ser. No. 321,159 Claims priority, application Sweden, Nov. 9, 1962,

3 Claims. (Cl. 51-288) The present invention relates to a method for sharpening rock drill bits especially of the kind having crosswise arranged cutting inserts. In rock drill bits of this type it has hitherto been usual to resharpen the cutting inserts by a separate grinding of each of the rearwardly inclined surfaces which form the cutting edges. In fouredged drill bits this involves eight separate grinding operations. By the invention it is possible to grind two edge surfaces at a time, whereby the number of grinding operations for four edged bits is reduced to four. In order to make this possible a grinding wheel with an annular grinding surface is used, the drill bit being placed with one edge or insert thereof within the recessed center portion of the wheel and the two adjacent edges in simultaneous contact with the grinding surface.

The method according to the invention is characterized in that the grinding is performed with a grinding wheel having an annular, preferably plane grinding front surface and a recess at the center of said surface, the drill bit being placed with one edge thereof extending into ing drawings in which:

FIG. 1 is a view on the line A-A of FIG. 2, and FIG. 2 is a side elevational view, partly in section, of a grinding apparatus and of a percussion drill bit in position for grinding.

The hub of the grinding wheel is attached to a driving shaft 11, which is carried on ball bearings and driven by a belt transmission 12 by a motor (not shown). The grinding Wheel is provided with an annular grinding plate 30 having a grinding layer 13, which has a preferably plane grinding surface. The grinding surface may be somewhat conical or curved. The grinding layer consists preferably of diamond grains embedded in metal. For non-electrolytic grinding the layer must be abrasive. In general the layer is abrasive also in electrolytic grinding, but it has also been found possible to use a non abrasive surface for electrolytic grinding. The ring 30 is fastened to the hub 10 by fastening means such as screws (not shown). The grinding wheel has at its center portion a recess 14 in order to provide room for one of the bit edges or inserts not being ground. The recess can for instance have a cylindrical shape but can also advantageously be widened from the opening and inwardly as illustrated. On the rear side of the grinding wheel there is a contact layer 15, against which a resilient electrical contact brush device 16 rests. A drill bit 17 having four inserts of sintered carbide is shown in simultaneous contact with the grinding layer 13. From a tube 21 an electrolyte 22 is flushed towards the grinding layer. A direct current source 23 has its positive pole connected to the drill bit 17 and its negative pole to the brush 16. The hub 10 and the ring 30 should preferably be made of metal in order to provide, electrical conductivity between the contact layer 15 and the grinding layer 13.

When the grinding wheel is rotated and electrolyte is flushed onto the grinding layer, the electrolyte is brought into the space between the layer 13and the surface to be ground on the inserts 19 and 20. Under the influence of the source of current 23 material is'electrolytically removed from the anode, i.e. the hard metal inserts 19 and 20. If the grinding layer consists of diamonds embedded in metal, the diamonds have the twofold function of removing the material disintegrated by the electrolysis and of maintaining a determined small dis tance between the drill bit and the grinding layer, so that electrolyte can pass between the bit and the layer. Short-circuiting also is avoided. Any direct abrasive effect of the diamond grains is thus not used for the grinding, the grinding being performed by the electrolyte. The speed of disintegration is dependant on the current which can be regulated by any suitable device such as a transformer 24 that is adjustable in steps. The current is limited by the insulation provided by the diamond grains, and the voltage must be kept below the limit where sparking and especially arcing begins. A certain amount of sparking can often not be avoided and has therefore to be allowed. The composition of the electrolyte has an influence upon the electrical conductivity and consequently also upon the speed of disintegration but also such factors as aggressivity, surface fineness etc. have an influence upon the choice of electrolyte. The source of current is usually a rectifier, which can be manually or automatically adjustable.

When the diamond grains in the wheel are worn down it is possible to reverse the poles in the electric circuit, which results in a disintegration of metal in the surface of the grinding layer thus exposing more of the diamond grains.

The invention can be used not only for electrolytic grinding but also for conventional grinding by shutting oif the electric current and the supply of electrolyte but this will give rise to a substantially increased wear of the grinding wheel. The invention can be used not only for sintered metal inserts but also for cutting edges and cutting inserts of other materials. The invention is preferably used for percussion drill bits having four cutting edges, said edges usually being placed at right angles to each other, but the invention can be used for any bit having one or more pairs of diametrically opposed cutting edges.

For electrolytic grinding according to the invention it has been found suitable to use a current of 50-150 amperes per square centimeter ground surface and a voltage of 5-12 volts. As electrolyte is used a solution of suitable salts for obtaining electrical 'conductibility and also corrosion protection, as potassum nitrate, sodium nitrite (corrosion inhibiting), sodium carbonate, potassium carbonate and potassium-sodium tartrate. Suitable solutions are for instance, the solvent being water: (I) 4% potassium-sodium tartrate, 4% potassium nitrate, 0.7% sodium nitrite, 0.3% potassum carbonate. (II) 4% sodium carbonate, 4% sodium nitrite. (III) 12% potassium nitrate, 4% sodium nitrite.

For grinding a drill bit having 7" diameter a grinding wheel can be used that has an abrasive surface of diamonds embedded in steel at a density of 2.5-4.5 carats/cm. (35-70 carats/inch and a grain size below 0.5 mm. The outer diameter is 250 mm. and the inner diameter mm., and the velocity 2000-2500 r./min. The bit is pressed against the wheel with a specific pressure of 5-10 kg./cm.

o r V d I I claim: 7

j 1.' Method of grinding'a rock drill bit having at least two spaced apart, alined, straight'cutting edges at least a third cutting edge lying in substantially the same plane;

said surfaces with a plane "annular grinding disc having a central recess with said third cutting edge extending int0'saidrecess.- a I 7 '2. Method as defined 'in clairnl in'- which the recess in the grinding wheel is widened "towardj the bottom thereof.

5 i 3. Method as defined inclaiml inwhich the grinding is performed electrolytically. l

V ReferencesCited by theExaminer 1 UNITED STATES PATENTS 2,326,521

ROBERT C. lRIo oN; Primal- E xain iner, LESTER M. WI GLEQ Examiner. 

1. METHOD OF GRINDING A ROCK DRILL BIT HAVING AT LEAST TWO SPACED APART, ALINED, STRAIGHT CUTTING EDGES, AT LEAST A THIRD CUTTING EDGE LYINT IN SUBSTANTIALLY THE SAME PLANE AS SAID TWO CUTTING EDGES AND EXTENDING AT AN ANGLE TO THE LINE THEREOF AND HAVING ITS END WHICH IS NEAREST TO THE LINE OF SAID TWO CUTTING EDGES SPACED FROM THE ADJACENT ENDS THEREOF, EACH OF SAID TWO CUTTING EDGES HAVING SUBSTANTIALLY PLANE, REARWARDLY EXTENDING SURFACES ADJACENT THERETO, WHICH COMPRISES CONTACTING TWO OF SAID SURFACES WITH A PLANE ANNULAR GRINDING DISC HAVING A CENTRAL RECESS WITH SAID THIRD CUTTING EDGE EXTENDING INTO SAID RECESS. 